In the field of semiconductor manufacturing, ion implantation is a basic unit operation of semiconductor device fabrication. Ion implantation equipment may be of widely varying type, and may include beam ion implant systems, plasma immersion systems, and systems of other varied types.
In the use of beam ion implant systems, positively charged ions impinge on the wafer substrate being implanted, and this impingement may lead to build-up of positive charge on insulated regions of the wafer substrate, producing positive surface potentials. Wafer charging may also result from secondary emission of electrons from the wafer substrate. The wafer substrate surface charges may be sufficiently strong to adversely impact or even permanently damage integrated circuitry features of the wafer such as metal-oxide-semiconductor field-effect transistor (MOSFET) circuits.
Plasma flood gun apparatus can be used to address such surface charge build-up, by generating plasma comprising low-energy electrons, so that the low-energy electrons can be dispersed into the ion beam and transported to the wafer surface to neutralize the charge build-up that would otherwise occur.
Plasma flood gun apparatus may be of varying types, but characteristically comprise an arc chamber arranged with an ionization filament element and coupled to a plasma tube circumscribed by solenoid coils, and communicating with an ion beam chamber. The ionization filament element in the arc chamber is formed of a refractory metal, often tungsten, and the gas used to form the low-energy electron plasma is characteristically an inert gas such as argon, krypton, or xenon, among other possibilities. A Faraday assembly may be included for confinement of the neutralizing electrons to the vicinity of the wafer, to thereby assist in mitigating wafer substrate charging, and typically include electron dose, uniformity, and charge measurement and monitoring components.
Thus, plasma flood gun apparatus address operational issues in beam ion implant systems, functioning to neutralize the beam plasma charge to control particle raisings, and reducing charge-up voltage on wafer substrates to prevent electrostatic destruction of thin film integrated circuitry elements.
During plasma flood gun operation, charge-neutralizing low-energy electrons are introduced into the ion beam and act on a substrate wafer to neutralize positive charges that build up on a wafer. However, during this process, inert gas can incidentally sputter the plasma flood gun filament and slowly cause its deterioration. The sputtered filament material becomes a gaseous material that can become deposited onto insulators and graphite components of the ion implant system as deposited contaminants. More generally, with extended operation, ion beam and condensable gas vapors deposit in, on, and around the plasma flood gun arc chamber, and its components. Such vapors also deposit on the Faraday (dose measurement) assembly to which the plasma flood gun is electrically coupled. Those deposits, regardless of their specific origin, are detrimental to the performance of the plasma flood gun system, and are detrimental to the operating lifetime of the system. In terms of performance, for example, these deposits are prone to result in electrical failure due to electrical shorting. Also relating to performance, sputtered filament material, e.g., tungsten, can make its way as into a wafer substrate being ion implanted, placing the sputtered filament material e.g., tungsten, as a contaminant in the substrate and reducing product yield of an ion implantation system and process.
These deposits can also decrease plasma flood gun emission currents, increase filament leakage currents, and, because the plasma flood gun is part of the dose measuring system, create Faraday leakage currents. All of these effects of deposited contaminants within an arc chamber of a flood gun can have a cumulative effect during operation, in a manner that can require regular maintenance, including cleaning of the deposited contaminants, and that can over time reduce the effective lifetime of a plasma flood gun.